Thread: Duel Battery Solenoid

When you house battery is drained and you start the engine, the batteries will connect and drop a lot more than 140 Amps across.

When you house battery is drained and you start the engine, the batteries will connect and drop a lot more than 140 Amps across.

But that is only between batteries and they are giant capacitors which will smooth any fluctuations.

In practice I find that any transfer is not instantaneous but takes a while to happen and by that time you have started the car.

In fact when I look at my 2 volt meters it takes minutes for the batteries to equalize. BTW, I never let my 120AH AGM get below 11.5 and the only load is the Engel which has a cutout at 11.5.

The bigger surge on a D2 is when the fuel pump activates and the capacitors in the ECU spool up. Maybe glow plugs but no light. This is worth about one volt from the starting battery and I only usually start after the voltage has recovered from say 11.6 to 12.4 or so.

I still haven't figured what the similar surge is on shutdown but the starting battery drops a volt then also for a few seconds. Again maybe glow plugs cleaning or something.

Memo to self .Don't forget to turn off the ignition. Never have done that.

In fact I have another don't forget as I have my 20/20amp DC charger and 12v element fridge in the van wired with the accessory position through a switch and 200amp relay. However I would have to consciously decide to leave acc on to maybe charge my van lithium from a solar panel via the second battery as well as leave the car connected to the van.

Regards PhilipA

Originally Posted by Red90

When you house battery is drained and you start the engine, the batteries will connect and drop a lot more than 140 Amps across.

The 50 amp circuit breakers supplied with my isolators, easily hand the start up currents required to start any motor.

I have many customers who have jump started through 50 amp auto resetting circuit breaker and the circuit breakers do not trip while starting.

It is not just the amount of current that is required to trip a circuit breaker or fuse, but the time that current is applied to a circuit.

As this high current is only applied for a few seconds at most, 50 amp fuses or circuit breakers are fine.

I give up. You are not understanding. I’m out.

Originally Posted by Red90

I give up. You are not understanding. I’m out.

I understand what you are talking about, and I should have posted up more info.

But as the second battery is going to be in the rear of the vehicle, even if it had dropped a cell, the maximum current draw would be around 70 amps for a very large faulty auxiliary battery.

And this is why a I used jump starting as an example, because it does draw more than a flat auxiliary battery.

The maximum current draw, for a very low 100Ah AGM would be 60 amps for a short time, tapering off from there.

Originally Posted by Red90

When you house battery is drained and you start the engine, the batteries will connect and drop a lot more than 140 Amps across.

For decades the biggest “market supplied” DBS were rated at a max of 100a.

Example of Traxides 90 unit - rated 90a

It’s not a problem.

Originally Posted by Red90

I give up. You are not understanding. I’m out.

Unfortunately I’d say it’s you who isn’t quite understanding.

Originally Posted by Tombie

For decades the biggest “market supplied” DBS were rated at a max of 100a.

Example of Traxides 90 unit - rated 90a

It’s not a problem.

Yep, I use to have a 45 amp isolator, the SC40, and as alternators got bigger and bigger, the SC40 did prove to be too small and production of the SC40 ceased about ten years ago.

My SC80 ( a 90 amp isolator ) and the DT90 also a 90 amp isolator work fine in all situations but one.

These two isolators are not designed and are not suitable for winch applications and this is why I introduced my USI-160 isolator a little over a decade ago.

While both the SC80 and DT90 have a CONSTANT current load capability of 90 amps they regularly handle much higher currents for short periods of time without any problems.

I have been experimenting with lithium auxiliary batteries for some time now and they will provide upwards of 150 amps while starting a motor, and the DT90 not only handles this without issue, I have also been experimenting with different size fuses.

I began with 80 amp fuses and every so often I would reduce the fuse size.

At present, I am running my 100Ah lithium through a 50 amp fuse and even with the high starting currents, the 50 amp fuse has not blown once.

NOTE, the cranking battery is under the bonnet and the Lithium is in the boot, connected via 6B&S ( 13.5mm2 ) twin cable and the vehicle has a 140 alternator.

Alls well so far!